2.1 Study design
The CHNS is an ongoing open cohort, an international collaborative project between the Carolina Population Center at the University of North Carolina at Chapel Hill and the National Institute for Nutrition and Health (NINH, former National Institute of Nutrition and Food Safety) at the Chinese Center for Disease Control and Prevention (CCDC). It was designed to examine the effects of the health, nutrition and family planning policies and programmes implemented by national and local governments and to see how the social and economic transformation of Chinese society affects the health and nutritional status of its population. The longitudinal CHNS has been conducted since 1989 in eight out of the 23 Chinese provinces (Guangxi, Guizhou, Henan, Hubei, Hunan, Jiangsu, Liaoning, Shandong), Heilongjiang province was enrolled as a ninth province in 1997.[11] In 2011, three megacities (Beijing, Shanghai, Chongqing) agreed to participate in the study, whose results vary substantially in terms of geography, economic development, public resources and health indicators.[12] A multistage, random cluster process was used to draw the sample in each of these provinces. Counties and cities in each province were stratified by income (low, middle and high), and a weighted sampling scheme was used to randomly select four counties and two cities in each province. Villages and townships within the counties and urban and suburban neighbourhoods within the cities were selected randomly. In each community, 20 households were randomly selected and all household members were interviewed.[11]
2.2 Participants
In 2011, data of 15,732 participants were merged. Data were collected from participants during face-to-face interviews with trained interviewers. Exclusion criteria include missing weight or height information (n=629); below 6 years of age and over 17.99 years of age (n=13,531); unknown values on the primary variables of TV food ads related purchasing behaviours, ethnicity and education (n=33); missing values on the primary variables of age, education, urbanisation index, per capita annual family income, ethnicity, TV food ads related purchasing behaviours, dietary intake of energy, fat, protein and carbohydrates (n=122). The final sample included in the analysis was 1,417 children aged 6–17.99 years.
2.3 Measures
2.3.1 Socio-demographic characteristics. Age, education, urbanisation index and per capita annual family income were encoded and converted into count data. Self-reported education was recoded into three categories (illiterate/primary school, junior middle school, high middle school or higher).[13] The urbanisation index and per capita annual family income were recoded into tertiles (low, medium, high).[13]
2.3.2 TV food ads related purchasing behaviours. TV food ads related purchasing behaviours were assessed by the questions “do you ask your parents to purchase the foods seen on TV ads?”, “do your parents purchase advertised foods for you?” and “do you purchase the foods seen on TV ads by yourself?”. Respondents reported the frequency (“very seldom”, “seldom”, “sometimes”, “often”, “very often” and “unknown”) of these purchasing behaviours. The options of “very seldom”, “seldom”, “sometimes”, “often” and “very often” were defined as “<1 time/month”, “1–3 times/month”, “1–2 times/week”, “3–4 times/week”, “≥5 times/week”, respectively. For each of the above purchasing behaviours, responses of ‘very seldom’ and ‘seldom’ were combined as ‘no’; and responses of ‘sometimes’, ‘often’ and ‘very often’ were combined as ‘yes’ for the analysis.
2.3.3 Dietary intake. Individual dietary data for the same three consecutive days were recorded for all household members, regardless of age or relationship to the household head.[12] This was achieved by asking each individual, except children aged younger than 12, on a daily basis to report all food consumed at home and away from home on a 24-hour recall basis.[12] If the children were not present, their caregivers would be required to contact them to obtain the children’s food consumption information. For children younger than 12, the mother or a mother substitute who handled food preparation and feeding in the household was asked to recall the children’s food consumption.[12]
2.3.4 Overweight and obesity. Height and weight were measured in the 2011 CHNS survey. Height and weight of children were measured by at least two trained health workers who followed standard protocol and techniques, with one worker taking the measurements while a second health worker recording the readings[14]. Body weight was measured in light indoor clothing to the nearest tenth of a kilogram with a beam balance scale; height was measured without shoes to the nearest tenth of a centimeter, using a portable stadiometer[14]. The International Obesity Task Force cut-off of body mass index was used for defining overweight/obesity among children.[15]
2.4 Data analyses
Descriptive statistics were used for the sample characteristics. The categorical variables were described using frequency and percentile, and the metrological data description used mean and standard deviation. Student’s t-test was performed to examine the relationships between TV food ads related purchasing behaviours and dietary intake of energy, fat, protein and carbohydrates. To determine if the relationships between TV food ads related purchasing behaviours and overweight and obesity existed, odds ratio (OR) and 95% confidence intervals (CI) for the outcome variable were calculated by using binary logistic regression. All statistical tests were conducted using STATA software (Version 12, StataCorp, College Station, TX, USA). Statistical significance was considered when P < 0.05 (two-sided).